Page 128 - Geology of Carbonate Reservoirs

DEPOSITIONAL ENVIRONMENTS AND PROCESSES 109

the petrophysical properties of small, separate vugs. If they are touching and are
not fi lled with cement, they may present high permeability values at comparatively
low porosity. A useful procedure is to plot permeability as a function of porosity
(log k vs φ ) when the data are available. This simple plot provides a great deal of
insight into the relationship between φ and k , especially if it is used together with
accurate rock descriptions.
Depositional porosity in reef organisms consists of intraparticle or intraskeletal
pores. Interparticle porosity exists mainly between skeletal constituents and between
detrital grains derived from the breakdown of the reef structure by hydrologic
action or bioerosion. Reef detritus may be gravel or sand sized and comprises the

loose fill between skeletal framework elements in reefs, around the perimeter of
the reef mass, or between reef masses. Depositional porosity is commonly higher in
the detrital fill than within the skeletal framework. Reef rock classifications, as we

have discussed, focus on the different textures and fabrics of reefs and carbonate
mounds. These schemes can be helpful to identify pore types and to map the spatial
distribution of specific pore categories, but they may not be useful in predicting

reservoir performance. Reef rock petrophysical properties are particularly complex
because the rock fabrics represent skeletal anatomy, oriented growth fabrics, and

detrital textures. Each rock type in a specific reef will have a predictable range of
pore characteristics, however. Because of the variability in reef rock texture and
fabric, reef reservoirs must be evaluated case - by - case. There is no single classifi ca-
tion that is universally applicable to predict reef reservoir characteristics.

5.2 DEPOSITIONAL ENVIRONMENTS AND PROCESSES

Most carbonate reservoir rocks are marine in origin; consequently, identifi cation of
individual depositional environments is a matter of dividing the marine environ-
ment into smaller sectors that have enough distinctive attributes to stand alone as
discrete subenvironments. Coastal dunes are included because they are usually part
of the beach and nearshore marine environment. Lacustrine carbonates are common
in the geological record, but they are not usually hydrocarbon reservoirs although
nonmarine carbonates may contain large quantities of hydrocarbons as “ oil shales. ”
According to North (1985) , nearly half of the world ’ s exploitable oil - shale resources
are in lacustrine carbonates of the Eocene Green River Formation. Green River oil
2
shales cover about 42,000 km in Colorado, Utah, and Wyoming. Rather than shales,
the Green River rocks are actually thin - bedded, bituminous limestones but some

zones in the Green River Formation are known for their fossil fish remains rather
than their bitumen content. Nonskeletal carbonate grains such as pisoids and oncoids
are present in Green River rocks, and some shoreline deposits around the Great
Salt Lake in Utah consist of oolite grainstones. For the most part, however, carbon-
ate reservoir rocks formed in the marine environment. Nonmarine reservoirs are
also known in mainland China, but there is not much published information on them
in English.
Oceanographers have already used arbitrary water depth to divide the marine
environment into ecological zones such as the neritic, bathyal, and abyssal zones.
But these depth zones cannot be identified in the rock record. Identifi cation of

ancient depositional environments requires the ability to recognize them by their